It has been known in the art to biaxially stretch films. Additionally, several methods and apparatuses have been described for biaxially stretching films simultaneously in two directions. See, e.g., U.S. Pat. Nos. 2,618,012; 3,046,599; 3,502,766; 3,890,421; 4,330,499; 4,525,317; and 4,853,602.
Tenters have been used for the transverse direction stretching in sequential biaxial film stretching processes. For a simultaneous biaxial stretching process, tenter stretching is performed on a tenter apparatus that has grips or clippers that grasp the film along the opposing edges of the film and propels the grasping means at varying speeds along guiding means, which typically are rails. As used herein, “grippers” and “clips” include other film-edge grasping means, and the word “rails” includes other clip guide means. By increasing clip speed in the machine direction, stretching in the machine direction occurs. By using such means as diverging rails, transverse direction stretching occurs. Such stretching can be accomplished, for example, by the methods and apparatus disclosed in U.S. Pat. Nos. 4,330,499 and 4,595,738, in which each of the clips is mechanically driven in the tenter apparatus. More recently, tenter frames for stretching films have been described in which the clips that propel the film through the tenter apparatus are driven by linear motors. See, e.g., the methods and tenter apparatus disclosed in U.S. Pat. Nos. 4,675,582; 4,825,111; 4,853,602; 5,036,262; 5,051,225; and 5,072,493.
In the simultaneous biaxial stretching apparatus described in U.S. Pat. No. 5,051,225, tenter clips are driven by linear electric motors. For reasons of spacing and cost, tenters such as described in the '225 patent may not have every clip driven by a linear motor. For example, every third clip on each rail may be driven by a linear motor with the intervening two clips being nondriven, and thus propelled forward only by the film itself. Such nondriven clips are referred to as idler clips. It has been observed that the relative position of the idler clips to the driven clips is not necessarily the ideal position of being equally spaced between driven clips. Any inequality in the clip-to-clip spacing among two nearest-neighbor driven clips on a rail and their intervening idler clips may be referred to using such terms as idler non-uniformity, uneven clip spacing, non-uniform clip spacing, and the like. Two special cases, however, are important. The case in which the first and last (or only) idler clip(s) between a pair of driven clips on a rail are propelled forward by the film in an amount less than would be necessary for equal spacing among clips is referred to as idler lag or lagging. The case in which the first and last (or only) idler clip(s) between a pair of driven clips on a rail are propelled forward by the film in an amount greater than would be necessary for equal spacing among clips is referred to as idler lead or leading. In the case where there is more than one idler clip between each pair of driven clips on each rail, it is possible to have one propelled forward by the film in an amount less than would be necessary for equal spacing among clips and, simultaneously, to have the other propelled forward by the film in an amount greater than would be necessary for equal spacing among clips. This situation results in an uneven clip spacing, or idler non-uniformity, which is neither an idler lag nor an idler lead.
U.S. Pat. No. 5,753,172 describes a process for the simultaneous biaxial stretching in a tenter frame of a thermoplastic polymer film having beaded edges, comprising gripping the beaded edges of the film with tenter clips and increasing the temperature of the beaded edges to within the film orientation temperature range prior to or during simultaneous stretching, and in subsequent stretching or heat-setting steps, by focusing heat on the beaded edges of the film. The '172 patent states that bead temperatures that are either too high or too low or beads that are too thin can cause the spacing of the idler clips to be non-uniform. Column 3, lines 30-33; column 1, lines 58-62. The '172 patent further states that it is generally desirable for the temperature of the beads to be approximately equal to, or higher than, the temperature of the central film web. Column 5, lines 27-29. The '172 patent also states that the need for separate control of bead temperatures is driven in part by the unequal heating applied to the beads compared to the film in typical stretcher heating zones. Col. 5, lines 33-35. It is both well-known in the art and demonstrated in the '172 patent (Col. 11, lines 35-40) that such unequal heating in typical stretcher heating zones leads to the beads being cooler than the central film web. U.S. Pat. Nos. 3,231,642; 3,510,552; and 5,429,785 also discuss certain effects of temperature control in various film stretching processes.